Previous investigations of testosterone and externalizing behavior have provided mixed findings. We tested the hypothesis that self‐regulatory personality moderates the testosterone–externalizing behavior association in adolescence. Parents reported on their 13‐ to 18‐year‐old (N = 106, Mage = 16.01, SD = 1.29) children's personalities and psychopathology. Testosterone was measured via drool samples. As hypothesized, personality moderated the testosterone–externalizing behavior association. High testosterone predicted higher levels of externalizing behaviors, but only for adolescents low in Agreeableness and Conscientiousness. Also, personality acted as a resiliency factor: high levels of Conscientiousness, in the presence of high testosterone, predicted lower levels of rule breaking. Results highlight how endogenous factors, such as personality, may interact with testosterone, and emphasize the relevance of including personality moderators in future research.